1. Preheat oven to 350°F (176°C) and prepare a cupcake pan with cupcake liners.
2. In a large mixing bowl, cream sugar, butter and peanut butter until light in color and fluffy, about 3-4 minutes.
3. Add sour cream and vanilla extract and mix until well combined.
4. Add egg whites in two batches, mixing until well combined after each. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated.
5. Combine dry ingredients in another bowl, then combine milk and water in another bowl. Add half of the dry ingredients to the batter and mix until well combined. Add the milk mixture and mix until well combined. Add remaining dry ingredients and mix until well combined. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated.
6. Fill cupcake liners about halfway. Bake for 15-17 minutes, or until a toothpick inserted comes out with a few crumbs.
7. Remove cupcakes from oven and allow to cool for 2-3 minutes, then remove to cooling rack to finish cooling.
8. While cupcakes are cooling, make the frosting. Beat the butter, shortening and Nutella together until smooth.
9. Add about half of the powdered sugar and mix until smooth.
10. Add the water or cream and mix until smooth.
11. Add the remaining powdered sugar and mix until smooth. Set frosting aside.
12. Once cupcakes are completely cool, cut out the centers. I use a cupcake corer.
13. Fill the centers with marshmallow cream/fluff.
14. Pipe the frosting on top of the cupcakes. I used Ateco tip 844, but the Wilton 2D or 1M would create a similar look.
15. To decorate the cupcakes, top each with a large marshmallow, the drizzle with chocolate sauce.

Store cupcakes in an air tight container at room temperature. Cupcakes are best for 2-3 days. Best to add chocolate sauce drizzle decoration shortly before serving.